1. Field of the Invention
The present invention relates to a light or optical subscriber network system, and in particular to an optical subscriber network system which is formed of a passive optical network.
2. Description of the Related Art
To subscribers requiring a high-speed and large capacity of communication, as shown in FIG. 12, has generally known a network system which accommodates optical subscribers #1-#n with optical network units generally designated by ONU through optical fibers generally designated by OF correspondingly constructed from an optical service unit (station unit) OSU in a station ST which is formed of the optical service unit OSU and a switchboard EX.
On the other hand, in recent years, a passive optical subscriber network system as shown in FIG. 13 has been put into practice which accommodates optical subscribers which need a broad-band communication.
This passive optical subscriber network system has an optical star coupler SC which consists of passive elements between a station unit and the optical network units (subscriber units). A pair or two pairs for redundancy of optical fibers OF1 are provided between the station ST and the star coupler SC. Optical fibers OF2, which are exclusive for each subscriber, are provided from the star coupler SC to the subscribers #1-#n.
In the star coupler SC, a down optical signal is distributed from the station ST to the respective optical subscribers #1-#n while up optical signals are united from the optical subscribers #1-#n to the station ST.
In such an optical subscriber network system as mentioned above, TCM (Time Compression Multiplexing) method has been used which adopts TDMA (Time Division Multiple Access) method enabling subscribers to access the station unit by allotting time slots to the up signal from the optical network unit ONU to the optical service unit OSU and TDM (Time Division Multiplexing) method used in the down signal from the optical service unit OSU to the optical network unit ONU when the outputs of the plural optical network units ONU are multiplexed to a single optical transmission line (optical fiber).
That is, as shown in FIG. 14, the down signal frame from the optical service unit OSU is provided for the optical subscribers e.g. #1 and #2 by the TDM method, and the up signal frame from the subscribers #1 and #2 to the optical service unit OSU is provided by the TDMA method.
In this TCM method, the construction cost of the system can be reduced by sharing both the optical transmission line and the optical service unit of the station, in comparison with the method of connecting the optical service unit OSU and the optical network unit ONU correspondingly. Moreover, the reliability of the system can be improved because passive elements are used for a star coupler, compared with the method of multiplexing/demultiplexing optical signals with an active unit.
In the passive optical subscriber network system shown in FIG. 13, it is considerably easy to form duplexed optical transmission lines for redundancy from the station to the star coupler.
However, the formation from the star coupler to the subscribers is not duplexed. Therefore, if a fault occurs in the optical transmission-reception portion (transceiver system) of the optical network unit ONU, the communication between the station and the faulted optical network unit ONU will be completely broken.
In the worst case, as shown in FIGS. 15 and 16, the optical transmission portion of the faulted optical network unit ONU2 emits light abnormally at the time slot TS' which is not the same as the time slot TS (shown by a dotted line) in which light should be emitted. The abnormal light emission may collide with the up frame in the optical network unit ONU1 which is accommodated in the same star coupler. Eventually the communication can be obstructed and interrupted.
Moreover, the maintenance in the station is relatively easy while it is difficult to specify whether the faulted point resides in the transmission line or in the optical network unit because information of the optical network unit is not obtained at the side upon the fault. Therefore, fieldworks increase so that restoration time, material and human costs grow to remove the fault of the optical network unit.
As measures for these problems, another optical network system has been considered as shown in FIG. 17. This system is formed of a star coupler SC, an optical fiber OF2, and an optical transmission-reception portion of an optical network unit ONU which are respectively duplexed. This system must follow a great cost increase.